<template>
  <ul class="list">
      <li 
      class="item" 
      v-for="key of letters" 
      :key="key"
      :ref="key"
      @touchstart.prevent="handlerTouchStart"
      @touchmove="handlerTouchMove"
      @touchend="handlerTouchEnd"
      @click="handlerClick"
      >
        {{key}}
      </li>
  </ul>  
</template>

<script>
export default {
  name: 'CityAlphabet',
  props:{
      cities:Object
  },
  data (){
    return {
      touchStatus: false,
      startY: 0,
      timer: null
    }
  },
  computed: {
    letters (){
      const letters = []
      for(let i in this.cities){
        letters.push(i)
      }
      return letters
    }
  },
  updated (){
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
    handlerClick (e){
      this.$emit('change',e.target.innerText)
    },
    handlerTouchStart (){
      this.touchStatus = true
    },
    handlerTouchMove (e){
      if(this.touchStatus){
        if(this.timer){
          clearTimeout(this.timer)
        }
       this.timer = setTimeout(() =>{
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if(index >= 0 && index < this.letters.length){
            this.$emit('change',this.letters[index])
          }
       })
      }
    },
    handlerTouchEnd (){
      this.touchStatus = false
    }
  }
}
</script>

<style lang="stylus" scoped> 
    .list
        display: flex
        flex-direction:column
        justify-content: center
        position:absolute
        top:1.58rem
        right:0
        bottom:0
        width:.4rem
        .item
            line-height:.4rem
            text-align: center
            color:#00bcd7
</style>